Question: The scale on a map is 2cm : 1km. If two cities are 4cm apart on the map, what is the actual distance in km between the two cities?
Answer: The scale means that every 2cm on the map corresponds to an actual distance of 1km. A distance of 4cm on the map is the same as 2 $\cdot$ 2cm. The actual distance between the two cities is 2 $\cdot$ 1km, or 2km.